The singer was the subject of an advert on the platform which asked users if they would "rather slap Rihanna or punch Chris Brown".
The ad, which was for a game, appeared to reference Chris Brown's conviction for assaulting Rihanna in 2009 while they were dating.
Snap Inc says the ad "never should have appeared on our service".
Rihanna posted a lengthy comment on an Instagram Story saying she was "trying to figure out what the point was with this mess".
"I'd love to call it ignorance, but I know you ain't that dumb," she wrote.
The company's share price has fallen by more than 4% since Rihanna's statement.
A Snap Inc spokesperson said: "This advertisement is disgusting and never should have appeared on our service. We are so sorry we made the terrible mistake of allowing it through our review process.
"We are investigating how that happened so that we can make sure it never happens again."
Rihanna accused Snapchat of "intentionally" shaming domestic violence victims.
"This isn't about my personal feelings, cause I don't have much of them... but all the women, children and men that have been victims of DV in the past and especially the ones who haven't made it out yet.
"You let us down! Shame on you. Throw the whole app-ology away."
The advert was widely criticised online on Monday.
Snap, who has blocked the Would You Rather game from advertising on its platform, previously told Newsbeat the advert was published "in error" and had been removed immediately.
"The advert was reviewed and approved in error, as it violates our advertising guidelines. We immediately removed the ad last weekend, once we became aware," the company said.
Twitter users described it as "disgusting", while the daughter of former presidential candidate Hillary Clinton said it being approved was "awful".